1. Field of the Invention
The present invention relates to a two-element fθ lens with short focal distance for a laser scanning unit, and more particularly to a two-element fθ lens used for a laser scanning unit with a polygon mirror, which has a short focal distance to reduce the volume of the laser scanning unit.
2. Description of the Related Art
At present, a laser scanning unit (LSU) used by a laser beam printer (LBP) controls a laser beam scanning by a high-speed polygon mirror as disclosed in U.S. Pat. Nos. 7,079,171, 6,377,293 and 6,295,116 or TW Pat. No. I198966, and the principles of their operation are described below: a semiconductor laser emits a laser beam through a collimator and an aperture to form parallel beams. After the parallel beams pass through a cylindrical lens, the beams are focused to form a line image, and projected onto the high-speed polygon mirror. The polygon mirror includes a plurality of uniform and continuous reflecting mirrors disposed at or proximate to the focal point of the line image. The polygon mirror is provided for controlling the direction of projecting the laser beam, so that when the plurality of continuous reflecting mirrors are rotated at a high speed, the laser beam projected onto a reflecting mirror can be extended in a direction of the scanning direction at the same angular velocity and deviated from and reflected onto a fθ linear scanning lens. The fθ linear scanning lens (so called fθ lens) is installed next to the polygon mirror and can be either a single-element lens structure or a two-element lens structure. The function of this fθ lens is to focus a laser beam reflected from the reflecting mirror of the polygon mirror and projected onto the fθ lens into a circular spot (or an oval spot) that is projected onto a photoreceptor surface (or a photoreceptor drum, which is an image side) to achieve the requirement of the scanning linearity as disclosed in U.S. Pat. Nos. 4,707,085 and 6,757,088 or Japan Pat. No. 2004-294713. However, the traditional fθ lens of laser scanning unit LSU still has the following drawbacks in its practical applications:
(1) When a laser beam is reflected from a polygon mirror, the central of the laser beam projected onto a reflecting mirror of the polygon mirror is not aligned physically with the central rotating axis of the polygon mirror, so that the design of a fθ lens has to take the issue of a reflection deviation of the polygon mirror into consideration. In the prior art, an optical correction of a sub scanning direction is adopted to correct the optics of a main scanning direction, as disclosed in U.S. Pat. Nos. 5,111,219 and 5,136,418, and Japan Pat. No. 2756125. However, these optical correction methods for correcting the reflection deviation by adjusting the sub scanning direction appropriately require a longer focal length, and thus results in an increased overall volume of the laser scanning unit.
(2) A fθ lens having a longer focal length is generally used to make the diameter of a spot of the scanning light projected onto a drum in compliance with the specification or requirement of the application, in order to achieve a better image quality. Some manufacturers even use a reflecting mirror to extend the focal distance as disclosed in U.S. Pat. No. US2002/0063939 or a three-element lens as disclosed in U.S. Pat. Application No. 2002/0030158, and U.S. Pat. No. 5,086,350 and Japan Pat. No. 63-172217. Some manufacturers use a hard-to-make diffraction lens as disclosed in U.S. Pat. Application No. 2001/0009470 and U.S. Pat. No. 5,838,480. Some manufacturers use a two-element lens having an inflection point as disclosed in U.S. Pat. Nos. 5,111,219, 7,057,781, 6,919,993 or a single-element lens having an inflection point as disclosed in Japan Pat. No. 04-50908.
(3) One of the conventional methods for reducing the volume of the laser scanning unit to fit an application for a smaller printer is to shorten the focal distance to the drum as disclosed in U.S. Pat. No. 7,130,096, wherein the ratio of an effective scanning range and an image optical length is used for reducing the focal distance and eliminating the ghost image. As disclosed in U.S. Pat. No. 6,324,015, the restrained ratio (d/f) of the focal distance between the polygon mirror with the drum and the focal length of the fθ lens is used for shortening the focal distance. Thus, for example, the focal length is 100 mm, and the focal distance is approximately equal to 200 mm Moreover, a two-element fθ lens as disclosed in U.S. Pat. No. 6,933,961 adopts an asymmetric optical surface to produce an axis deviation on the main-scanning or the sub scanning direction, such that the optical axis of the two fθ lenses in the main scanning direction or the sub scanning direction in order to reduce the focal distance.
To satisfy consumer requirement for a light, thin, short and compact design of the laser scanning unit, a two-element fθ lens having a short focal distance (such as a focal distance less than 150 mm used in an A4 laser printer) may be adopted the needs in effective correcting optical distortion in the main scanning direction and the sub scanning direction as well as in improving the scanning quality and the resolution.